The structure-function relationships of the anti-tumor lectins abrin and ricin are being investigated by locating regions of amino acid sequence homology and correlating these with the biological activity of fragments containing these regions. A tryptic fragment of ricin has been shown to retain the full ability to inhibit protein synthesis in a cell free system. A homologous peptide is being sought in abrin. The complete sequence of abrin is being determined by automatic Edman degradations. This will be compared with the published sequence of ricin. Peptides prepared to encompass the homology regions will be tested for biological activity. Correlation of the data on biological activity with the amino acid sequence should help establish a structural basis for the design of chemotherapeutic agents with anti-tumor activity.